update help default options, add index

doc/nvim-tree-lua.txt

@@ -545,6 +545,7 @@ Following is the default configuration. See |nvim-tree-opts| for details. >lua
           warning = "",
           error = "",
         },
+        diagnostic_opts = false,
       },
       modified = {
         enable = false,
@@ -3179,6 +3180,7 @@ highlight group is not, hard linking as follows: >
 |nvim-tree.actions.use_system_clipboard|
 |nvim-tree.auto_reload_on_write|
 |nvim-tree.diagnostics.debounce_delay|
+|nvim-tree.diagnostics.diagnostic_opts|
 |nvim-tree.diagnostics.enable|
 |nvim-tree.diagnostics.icons|
 |nvim-tree.diagnostics.severity|